A well-known semiconductor memory component is random access memory (RAM). RAM permits repeated read and write operations on memory elements. Typically, RAM devices are volatile, in that stored data is lost once the power source is disconnected or removed. Non-limiting examples of RAM devices include dynamic random access memory (DRAM), synchronized dynamic random access memory (SDRAM) and static random access memory (SRAM). In addition, DRAMS and SDRAMS also typically store data in capacitors, which require periodic refreshing to maintain the stored data.
Recently resistance variable memory elements have been investigated for suitability as semi-volatile and non-volatile random access memory elements. In a resistance variable memory element, a conductive material, such as silver, is incorporated into a dielectric material. The resistance of the conductive material containing dielectric material can be changed between high resistance and low resistance states. The resistance variable memory element is normally in a high resistance state when at rest. A write operation to a low resistance state is performed by applying a voltage potential across the two electrodes.
One preferred resistance variable material comprises a chalcogenide glass. A specific example is germanium-selenide (GexSe100-x) containing a silver (Ag) component. One method of providing silver to the germanium-selenide composition is to initially form a germanium-selenide glass and then deposit a thin layer of silver upon the glass, for example by sputtering, physical vapor deposition, or other known techniques in the art. The layer of silver is irradiated, preferably with electromagnetic energy at a wavelength less than 600 nanometers, so that the energy passes through the silver and to the silver/glass interface, to break a chalcogenide bond of the chalcogenide material such that the glass is doped with silver. Another method for providing silver to the glass is to provide a layer of silver-selenide on a germanium-selenide glass. A top electrode comprising silver is then formed over the silver-germanium-selenium glass or in the case where a silver selenide layer is provided over a germanium-selenide glass, the top electrode is formed over the silver-selenide layer.
It has been found that over time devices fabricated via the above described methods fail if excess silver from a top silver containing electrode continues to diffuse into the silver germanium-selenium glass or into the silver-selenide layer and eventually into the germanium-selenide glass layer (the primary switching area) below the silver-selenide layer.
Furthermore, during semiconductor processing and/or packaging of a fabricated structure, which incorporates the memory element, the structure undergoes thermal cycling or heat processing. The memory element is also subject to heat during operation of the memory device containing the memory element. Heat processing can result in substantial amounts of silver migrating into the memory element uncontrollably. Too much silver incorporated into the memory element may result in faster degradation, i.e., a short life, and eventually device failure.
Control of the amount of available silver that enters the glass would be highly desirable to prevent premature memory cell failure.